Synthesis of germanium encapsulated fullerene

Article abstract of DOI:10.1016/j.ssc.2009.05.039

A method has been described for encapsulating Ge in a fullerene cage using GeO₂ and a graphite composite rod as anode and graphite as cathode. Annealing in an inert atmosphere before arcing, and the amount of GeO₂ in the rod determin

Full text of DOI:10.1016/j.ssc.2009.05.039

Solid State Communications 149 (2009) 1244–1247
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Solid State Communications
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Synthesis of germanium encapsulated fullerene
Debmalya Roya,∗, N.K. Tripathia, K. Rama, N. Sathyamurthyb,c
a Nanoscience and Technology Division, Defence Material and Store Research & Development Establishment (DMSRDE), DMSRDE P.O., G.T. Road, Kanpur-208013, India
b Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
c Indian Institute of Science Education and Research (IISER) Mohali, Chandigarh 160019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A method has been described for encapsulating Ge in a fullerene cage using GeO2 and a graphite composite
rod as anode and graphite as cathode. Annealing in an inert atmosphere before arcing, and the amount of
GeO2 in the rod determined the yield of Ge doped metallofullerene. Solvent extraction using soxlet in inert
atmosphere followed by calcination in air was used to isolate metallofullerene from the soot. The insertion
of Ge inside the fullerene was proven by ultra violet–visible absorption spectroscopy, matrix assisted laser
desorption ionization-time of flight mass spectroscopy, Fourier transform infrared spectroscopy, energy-
dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.
Received 4 March 2009
Accepted 22 May 2009 by P. Chaddah
Available online 31 May 2009
PACS:
81.05.Tp
Keywords:
A. Fullerenes
© 2009 Elsevier Ltd. All rights reserved.
D. Quantum localization
E. Atom, molecule and ion impact
1. Introduction
as aniline and pyridine by forming weak complexes with their
non bonding electrons. The open shell electronic configuration
Fullerenes have unique cage-like structures, which lead to the
typical inner space. The space inside fullerene is an exciting new
arena for experimental and theoretical studies [1–3]. A range
of metal atoms can be trapped inside this space to form endo-
hedral metallofullerenes. These new series of materials exhibit
potential applications as new type of superconductors, organic fer-
romagnets, nonlinear optical materials, functional molecular de-
vices, magnetic resonance imaging agents, and biological tracing
agents, etc [4,5]. A great deal of experimental and theoretical stud-
ies have been focused on endohedral metallofullerenes of group 3
metals (Sc, Y, La), most of the lanthanide series elements, group 2
metals (Ca, Sr, Ba), alkali metals (Li, Na, K, Cs), and some tetrava-
lent metals (U, Zr, Hf) [6]. However, no experimental and very few
theoretical [7,8] studies have so far been carried out on Ge, an in-
trinsic semiconductor, which has a comparable atomic size and
weight to that of metal ions inserted earlier in the fullerene cage.
The atomic radius of Ge is slightly smaller than other reported en-
dohedral metal atoms. However, it is big enough to get trapped in-
side the fullerene cage, which has typical 0.7 to 1 nm diameter on
and small highest occupied molecular orbital (HOMO)-lowest
unoccupied molecular orbital (LUMO) gap of metal encapsulated
in C60 leads to intermolecular polymerization rendering them
soluble in usual organic solvents. C82 endohedral metallofullerenes
are comparatively more soluble in common organic solvents and
thus more abundant in the isolated endometallo fullerene. Metal
encapsulated C82 has also open shell configuration, however due to
the significant electron density localization of unpaired electrons
inside the C82 fullerene cage hinder the intermolecular association
leading to solubilization in common organic solvents [10–12].
Confined metals inside the fullerene cage strongly interact
with the bonding and anti-bonding orbitals of fullerene and dras-
tically alter the electrical and conduction properties of pristine
fullerene [13–15]. Theoretical studies suggest that the electronic
properties of C60 could be modified by encapsulation of Ge (sup-
porting information). Molecular orbital analysis showed consider-
able mixing of LUMO of C60 with frontier orbitals of Ge resulting in
a new set of orbitals, which corroborate well with the experimental
photoluminescence spectra.
going from C60 to C240
.
C60 endometallo fullerenes are almost insoluble in common
organic solvents mainly due to their unusual reactivity towards
air and moisture and therefore very difficult to extract [4,9]. Metal
encapsulated in C60 are sparingly soluble in amine bases such
2. Material and methods
Synthesis of endohedral Ge doped fullerene was carried out in
electric arc reactor indigenously designed and developed in our
Laboratory. Ge impregnated 6 mm diameter graphite rod was used
as anode whereas pure graphite electrode of same diameter was
used as cathode. Composite graphite rod was prepared by mixing
∗
Corresponding author. Tel.: +91 512 2450404; fax: +91 512 2404774.
E-mail address: debmalya_roy@yahoo.com (D. Roy).
0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2009.05.039

D. Roy et al. / Solid State Communications 149 (2009) 1244–1247
1245
Table 1
Conditions of annealing and arcing of GeO2 and graphite composite rod as anode and pure graphite as cathode.
Condition of annealing
Vacuum (mbar)
0.028
Voltage (V)
10
Current (A)
65
Amount of He (mbar)
0
Distance between cathode and anode (mm)
0
Time (min)
45
Condition of arcing
0.023
38–40
95–100
135
5–6
5–6
graphite powder and GeO2 in a ball milling machine for 15 min at
400 rpm and then this mixture was filled through a 3 mm hole at
the centre of the graphite rod. The total Ge and carbon ratio was
maintained at 2.5:100. The composite graphite rod was annealed
before arcing in the presence of inert gas inside the arc ablator. The
details of annealing and arcing conditions are enlisted in Table 1 for
comparison.
The extraction of endohedral metallofullerene was carried out
by three steps solvent extraction using soxhlet under inert at-
mosphere at solvent boiling point. O-xylene was used first fol-
lowed by N, N dimethylformamide (DMF). Freshly distilled aniline
was then used to isolate metallofullerenes. The resulting solution
was filtered and solvent was evaporated on a rotary evaporator.
The sticky black material was thoroughly washed with acetone.
◦
The resulting black powder was heated below 200 C for 5 min
and then characterized by ultra violet–visible (UV–VIS) absorption
spectroscopy, matrix assisted laser desorption ionization-time of
flight (MALDI-TOF) mass spectroscopy, Fourier transform infrared
(FTIR) spectroscopy, energy-dispersive X-ray (EDX) spectroscopy
and X-ray photoelectron spectroscopy (XPS).
The mass spectroscopic analysis was performed by MALDI
technique using Bruker FLEX-PC2 ultraflex TOF mass spectrometer.
Measurements were carried out with 3, 5-Dimethoxy-4-hydroxy
cinnamic acid as the matrix. XPS was recorded on a Perkin Elmer
PHI 1257 X-ray Photo Electron Spectrometer. Ge and carbon core
level experiments were carried out using Al Kα (1486.6 eV photon
source) at different sputtering time (1×10−6 vacuum, 4 kV, 15 mA).
The UV–VIS spectra are recorded on Varian CARY 500 UV–VIS–NIR
spectrometer in spectroscopic grade DMF solvent using DMF as
standard reference.
Fig. 1. Schematic representation of the extraction procedure adapted for isolation
of Ge endometallo fullerenes.
oligomers of aniline and empty higher fullerenes. EDX analysis of
the annealed aniline extract shows the characteristic peak for Ge
(supporting information).
MALDI-TOF spectrum (Fig. 2) of annealed aniline extract shows
an intense peak around 1058 m/z indicates the abundance of
Ge@C82 in the product. A weak peak at 794 m/z indicates the
formation of Ge@C60, however due to aerobic condition of the
extraction procedure, the concentration of Ge@C60 is low in the
final isolated product. The presence of aniline in the annealed
aniline extract, confirmed by NMR spectroscopy (supporting
information), could be attributed to the formation of aniline adduct
with fullerene and metallofullerene during refluxing with aniline.
The peaks at 813, 887, 1151 m/z proves the formation of mono
3. Results and discussion
The percentage and distribution of metal in the composite elec-
trode are extremely crucial to get a higher yield of metallofullerene
in the soot [16,17]. The optimum percentage to get a Ge doped
monometallo fullerene was found to be around 2.5% by weight. The
uniform mixing of GeO2 with the graphite soot and tight packing of
the mixture into the cavity also play a positive role to obtain higher
percentage of Ge endometallo fullerene in the soot. Preheat treat-
ment of composite graphite rod before arc burning showed an en-
hancement of metallofullerene percentage in the starting soot. This
thermal activation process in the vacuum seems to convert GeO2
to GeC, which requires lower energy for bond cleavage (bond en-
thalpies of Ge–C and Ge–O are 460 and 659.4 kJ/mol respectively).
Details of the extraction process are schematically represented
in Fig. 1 Prior to soxhlet method, the starting soot was kept inside
acetone and then in dimethyl ether for three hours each to remove
amorphous carbon and other carbonaceous nanoparticles [4,6].
In o-xylene empty fullerenes, mainly C60 and C70, were extracted
whereas higher fullerenes and small amount of metallofullerenes
were soluble in DMF. MALDI-TOF analysis indicates no Ge endo-
metallo fullerene in the o-xylene extract and a very small amount
of metallofullerene in DMF extract (supporting information).
Aniline extract was washed thoroughly with acetone to remove
trapped aniline and oligomers of aniline. The black powder was
aniline adduct with C60, Ge@C60 and Ge@C82
.
Normally, the absorption spectra of metallofullerenes have long
tails to the red region which is also seen in Ge doped fullerene [18,
19]. The absence of any sharp peak at the UV region suggests the
insertion of Ge into the fullerene cage. However, the absorption
spectra of monometallo-fullerenes M@C82 (M = Sc, Y, La) showed
a sharp peak around 1000 nm [4]. This absorption peak has
been correlated to the intrafullerene electron transfer from the
encaged metal atom to the carbon cage. UV spectrum (Fig. 3) of Ge
doped fullerene shows a similar pattern like the other endohedral
metallofullerenes, however, absence of a sharp peak at 1000 nm
in the UV spectrum suggests a different distribution of electron
density in Ge doped fullerene compared to other endohedral
metallofullerenes.
The fitting of Ge core level XPS spectrum (Fig. 4) reveals three
peaks at 29.2, 30.5 and 33 eV, respectively. The peak at 29.2 eV
indicates that Ge is at zero valent state. A small shoulder at 27.9
◦
then annealed below 200 C in air to remove the unreacted aniline,

1246
D. Roy et al. / Solid State Communications 149 (2009) 1244–1247
170000
Ge@C82
3000
160000
2500
2000
1500
1000
500
150000
140000
130000
aniline adduct of Ge endometallo fullerene
C60
C70
Ge@C60
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
Binding energy (ev)
0
700
−6
Fig. 4. Ge core level XPS spectrum after 6 min of sputtering at 1 × 10 vacuum,
4 kV, 15 mA. The black thick line represents the 20 points FFT smoothing of data, the
scattered box symbol represents fitted spectrum and dotted lines represent three
fitted peaks at 29.2, 30.5 and 33 eV for Ge, GeO and GeO2 respectively.
800
900
1000
m/z
1100
1200
Fig. 2. MALDI-TOF spectrum of annealed aniline extract of Ge endometallo
fullerene. The intense signal at 1058 m/z indicates the abundance of Ge@C82 in the
isolated metallofullerene sample.
140000
130000
120000
110000
100000
90000
3
2
1
0
80000
70000
279 280 281 282 283 284 285 286 287 288 289 290 291
Binding energy (ev)
400
600
800
1000
1200
−6
Fig. 5. Carbon 1S XPS spectrum after 6 min of sputtering at 1 × 10 vacuum,
Wavelength (nm)
4 kV, 15 mA. The black thick line represents the 20 points FFT smoothing of data,
the scattered box symbol represents fitted spectrum and dotted lines represent two
fitted peaks at 284.4 and 286 eV for C–C and C–H bond respectively.
Fig. 3. UV–Vis–NIR spectrum of the annealed aniline extract of Ge endometallo
fullerene in spectroscopic grade DMF solvent.
eV confirms the interaction of fullerene core with Ge orbital [20].
The peaks at 30.5 and 33 eV are for GeO and GeO2 respectively.
It is already known that metal atom inserted into the fullerene
cage is very stable and cannot be oxidized by exposure to air
and moisture [20]. The presence of GeO and GeO2, therefore, are
attributed to the unreacted starting germanium oxide which also
extracted along with the metallofullerene.
and theoretical studies have been focused on endohedral metallo-
fullerenes of group III metals, most of the lanthanide series ele-
ments, group II metals, alkali metals and some tetravalent metals.
However no experimental study has so far been carried out on Ge
which has a comparable atomic size and weight to that of metal
ions inserted earlier in the fullerene cage. Ge has been encapsu-
lated in the fullerene cage by arcing anode and cathode made out
of Ge oxide-graphite composite and graphite, respectively. Heating
in an inert atmosphere and the percentage of GeO2 in the compos-
ite graphite rod are crucial in determining the yield and the nature
of Ge doped metallofullerenes. Solvent extraction and calcination
in air were carried out to isolate metallofullerene from the soot.
The various analytical techniques were used to characterize the Ge
endometallo-fullerene.
To confirm further that there is no Ge-fullerene exohedral
complex formation we have recorded the carbon 1S XPS spectrum
(Fig. 5). The presence of the peak at 284.4 eV for C–C bond and
the absence of characteristic peak at 283.8 eV [21] for Ge–C bond
are ascertained that Ge is caught inside the fullerene cage. FTIR
−1
spectrum below 400 cm
using CsI crystal does not show any
metal-carbon signal, this also furnishes additional confirmation
that Ge metal is inside the cage.
Acknowledgements
4. Conclusions
We would like to thank Mr. C. N. Ramachandran, IIT, Kanpur for
very useful discussions. The authors are grateful to the Director,
We discuss a method to encapsulate Ge, an intrinsic semicon-
ductor, inside the fullerene cage. A great deal of experimental

D. Roy et al. / Solid State Communications 149 (2009) 1244–1247
1247
DMSRDE, Kanpur for the permission to publish this manuscript.
We are thankful to Dr. Kingsuk Mukhopadhyay, DMSRDE, Kanpur
for his constant help and support. One of us (NS) thanks the
Department of Science and Technology, New Delhi for J.C. Bose
fellowship. We thank TCGA, Delhi for recording MALDI spectra. We
also thank Prof. A. Pradhan, IIT, Kanpur and Prof. Vikram Kumar,
NPL, Delhi for permitting us to record photoluminescence and XPS
spectra respectively. We express our thankfulness to Mrs. Debjani
Chattaraj and Mr. Amit Saraiya, DMSRDE, Kanpur for recording UV
spectra and for simulation of XPS spectra respectively.
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